Handle messages with only comments in the commit-msg hook

Commit messages with comments occur when the editor is aborted, and
should be treated as empty.

We do this using git-stripspace, which was introduced in 2005, so this
has no compatibility implications.

Change-Id: I78b50a789860cc11d63d891b0507786890158754

resources/com/google/gerrit/server/commit-msg_test.sh

@@ -26,6 +26,18 @@ function test_empty {
   fi
 }
 
+function test_empty_with_comments {
+  rm -f input
+  cat << EOF > input
+# comment
+
+# comment2
+EOF
+  if ${hook} input ; then
+    fail "must fail on empty message"
+  fi
+}
+
 # a Change-Id already set is preserved.
 function test_preserve_changeid {
   cat << EOF > input
@@ -113,7 +125,7 @@ EOF
 
 
 # Test driver.
-
+git init
 for func in $( declare -F | awk '{print $3;}' | sort); do
   case ${func} in
     test_*)

resources/com/google/gerrit/server/tools/root/hooks/commit-msg

@@ -27,17 +27,36 @@ if test ! -f "$1" ; then
   exit 1
 fi
 
-if test ! -s "$1" ; then
-  echo "file is empty: $1"
-  exit 1
-fi
-
 # $RANDOM will be undefined if not using bash, so don't use set -u
 random=$( (whoami ; hostname ; date; cat $1 ; echo $RANDOM) | git hash-object --stdin)
 dest="$1.tmp.${random}"
 
+trap 'rm -f "${dest}"' EXIT
+
+if ! git stripspace --strip-comments < "$1" > "${dest}" ; then
+   echo "cannot strip comments from $1"
+   exit 1
+fi
+
+if test ! -s "${dest}" ; then
+  echo "file is empty: $1"
+  exit 1
+fi
+
+if ! mv "${dest}" "$1" ; then
+  echo "cannot mv ${dest} to $1"
+  exit 1
+fi
+
 # Avoid the --in-place option which only appeared in Git 2.8
 # Avoid the --if-exists option which only appeared in Git 2.15
-cat "$1" \
+if ! git -c trailer.ifexists=doNothing interpret-trailers \
-  | git -c trailer.ifexists=doNothing interpret-trailers --trailer "Change-Id: I${random}" > "${dest}" \
+      --trailer "Change-Id: I${random}" < "$1" > "${dest}" ; then
-  && mv "${dest}" "$1"
+  echo "cannot insert change-id line in $1"
+  exit 1
+fi
+
+if ! mv "${dest}" "$1" ; then
+  echo "cannot mv ${dest} to $1"
+  exit 1
+fi